1. Field of the Invention
This invention relates generally to low-pass filters and, more particularly, to a reversible, odd-order low-pass microfilter for separating DSL and home networking signals from voice-band signals existing on the same pair of copper wires.
2. Description of the Background Art
With the advent of DSL (Digital Subscriber Line) and home networking data transmission technologies, it may be desirable to have DSL signals, home netvorking signals, or both present on a home telephone wiring network simultaneously with voice-band signals. Voice-band signals are commonly referred to as POTS (Plain Old Telephone Service) signals. Providing DSL service, home networking, and POTS over standard telephone lines permits the home telephone wiring network to operate as a local area network (LAN), while at the same time permitting voice-band and DSL service to be transmitted across the home telephone wiring network.
Despite the advantages of providing DSL, home networking, and POTS signals simultaneously over a common home telephone wiring network, it is desirable to prevent energy from the DSL and/or home networking signal carriers from reaching voice-band, or POTS, appliances coupled to the home telephone wiring network. It is also desirable to prevent POTS device impedance effects beyond about 4 kHz from entering onto the home telephone wiring network and disrupting transmission of DSL data signals. Voice-band appliances may include, for example, telephone sets, facsimile machines, 56K modems, and the like. Indeed, energy from the DSL or home networking signal carriers may cause nonlinear behavior of the voice-band appliances to create noise into the POTS connection. Further, preventing DSL and home networking signals from reaching voice-band appliances protects the DSL and home networking transports from high-frequency inter-modulation products of the voice-band appliances.
Voice-band appliances typically undergo impedance changes during operation. For example, state changes in a POTS device such as on/off hook, dialing, and ringing tend to affect the impedance of the POTS device. This change in impedance, unless isolated from the DSL modem, may limit the throughput of the DSL or home networking devices and may require dynamic bit reloading in modulation and line retraining, and could result in loss of modem connection.
Conventionally, a second-order low-pass Butterworth filter is disposed between the home telephone wiring network and an associated POTS device to prevent DSL signals, such as ADSL signals, on the home telephone wiring network from entering the POTS device and to prevent transient noise from POTS devices from interfering with the proper operation of a DSL modem coupled to the home network and vice versa. The filter topology of the second-order Butterworth microfilter is inherently asymmetrical and generally includes one coupled inductor (or two uncoupled inductors) and one capacitor. This design is unilateral and non-reversible in that it requires, for proper operation, that the microfilter be oriented between the POTS device and the home telephone wiring network such that the coupled inductor is disposed between the home telephone wiring network and the capacitor. Indeed, if the capacitor is disposed adjacent to the home telephone wiring network, high frequency signals, such as DSL signals, on the home telephone wiring network are likely to short, or be shunted, across the capacitor, thus interfering with the operation of the DSL modem. In short, these conventional microfilters are not reversible in that they only function properly when correctly oriented. Thus, users who install the two-pole microfilter in a reversed, or xe2x80x9cbackwardsxe2x80x9d, tit configuration will likely suffer from poor filter and DSL modem performance.
Another disadvantage of conventional second-order. Butterworth microfilter designs is that they do not provide sufficient attenuation of DSL. signals. For example, a typical second-order Butterworth microfilter may be designed with an insertion loss of about 0.3 dB loss throughout the pass band, which includes the POTS band (about 0-4 kHz) and has a cutoff frequency of about 8 kHz. As those skilled in the art will appreciate, it is highly desirable for this cutoff frequency to be above the POTS signal band and well below the ADSL transmission band (i.e. below about 25 kHz). Given the 8 kHz cutoff frequency, the total attenuation achieved at 25 kHz (the beginning of the DSL band) is, at a maximum, only about 19 dB. This amount of attenuation is generally insufficient in that it allows a significant amount of DSL transmit signal leakage through the filter, and could cause interference with the associated POTS device, particularly if the associated POTS device is a data device, such as a facsimile machine or a data modem.
Further, as with many things, it is desirable to keep the costs of producing the microfilter low. A significant factor in determining the cost of producing a microfilter is the number of components that make up the microfilter. In general, the higher the number of components that make up the microfilter, the higher the cost will be to produce the microfilter. Consequently, it is desirable to keep the component count of a given microfilter design low to keep the production cost low.
Accordingly, a need exists to provide a system and method for preventing energy from DSL and home network signal carriers from reaching voice-band appliances such as telephones, facsimile machines, and 56K modems. Another need exists to provide a system and method for isolating DSL devices and HPNA (Home Phoneline Network Alliance) standard devices from the impedance fluctuations of voice-band appliances. Moreover, an additional need exits to provide a system and method for separating, or isolating, voice-band appliances from DSL and HPNA devices that is robust, inexpensive, and easy to install.
The present invention overcomes or substantially alleviates prior problems associated with systems and methods for separating or isolating voice-band appliances from DSL and HPNA devices. In general, the present invention provides a reversible odd-order low-pass filter that may be disposed between a POTS device and a home telephone wiring network to isolate the POTS device from certain higher frequency signals, such as DSL band signals, that may be present on the home telephone wiring network. The filter also prevents POTS device impedance changes within the DSL band from appearing on the home telephone wiring network.
The filter topology is substantially symmetric in that the topology features reversible plug-in capability so that either end of the filter may be coupled to the home telephone wiring network without interfering with the filtering function of the filter and without shorting, or shunting, the higher frequency signals, such as ADSL signals, on the home telephone wiring network. Further, the filter presents aselatively low insertion loss so that the filter does not cause significant attenuation of the POTS signal as the POTS signal passes through the filter, thus not impairing the operation of the associated POTS device.
Pursuant to one embodiment, the low pass filter is a passive odd-order low-pass reversible microfilter that includes a capacitor disposed between pairs of inductors for disposition between the telephone wiring network and an associated POTS device to high frequency signals from reaching the POTS device and for isolating the POTS device impedance changes from the network. The inductors may be either coupled or uncoupled inductors. According to this configuration, the microfilter is reversible in that either end of the microfilter may be disposed adjacent to the to the telephone network without shorting high frequency signals, such as ADSL signals, across the capacitor due to the presence of a pair of inductors on both sides of the capacitors.
In another embodiment, the low-pass filter is a passive third-order low-pass microfilter design scheme based on a 600 ohm balanced network and includes a pair of coupled inductors separated by a capacitor. Advantageously, each of the coupled inductors has combined interwinding capacitance greater than about 100 pF. Due to the intentionally-introduced interwinding capacitance of the coupled inductors, the frequency response of the low-pass filter closely resembles that of a higher order Chebyshev II function, also known in the art as an xe2x80x9cinverse Chebyshevxe2x80x9d function This filter function generally has a flatter magnitude response in the pass band than conventional Chebyshev filters and much steeper attenuation beyond the cutoff frequency than the conventional Butterworth filter described above. Pursuant to this embodiment, the filter presents more than about 35 dB of attenuation beginning at 25 kHz with a 3 dB cutoff frequency point at about 6 kHz, which offers sufficient signal rejection to prevent DSL signals from reaching the associated POTS band device and potentially causing performance degradation.
This odd-order design is advantageous in that it effectively and inexpensively isolates high frequency signals, such as DSL signals, on a home telephone wiring network from reaching POTS devices, as well as preventing the impedance changes of the POTS devices from interfering with the DSL or home networking service. Additionally, since the odd-order filter does not employ a capacitor disposed outside of the coupled or uncoupled inductors, the filter is reversible in that either end of the filter may be coupled to the home telephone wiring network without shorting, or shunting, the high frequency signals across the capacitor or otherwise impairing the filter characteristic of the filter. Thus, even if a user installs the filter xe2x80x9cbackwards,xe2x80x9d the filter will still generate the same frequency response and function satisfactorily without shorting the higher frequency signals on the home telephone wiring network.
Further, the present design provides a microfilter having a single resonant frequency point disposed above the POTS frequency band (i.e. above about 4 kHz) and well below the DSL band (i.e. below about 25 kHz) to avoid introducing unexpected impedance impairments to both POTS and ADSL operations. Additionally, the microfilter provides a relatively high impedance across the DSL frequency band beginning at about 25 kHz for both on-hook and off-hook conditions to block impedance changes caused by an associated POTS device. Moreover, to avoid high signal loss across the DSL band, the microfilter advantageously has an input impedance of at least 400 ohms.